Researcher proves value of sex

STANFORD -- What's so great about sex?

Roughgarden simulated 5,000 generations of two hypothetical species to
solve a puzzle that has intrigued biologists for 50 years: on paper, asexual
species should have a big head start over species that reproduce sexually,
but in nature, sexual reproduction is the norm.

Roughgarden's simulation reveals the long-term evolutionary advantage of
sex - an advantage that overturns a common assumption of biology. It's not
the ability to generate new combinations of genes that makes sexual species
more resilient in case of change, he found. Sex triumphs because it preserves
relatively steady amounts of existing gene combinations.

From an ecological perspective, sex seems like a losing proposition. As
biologist Maynard Smith pointed out 20 years ago, if a population reproduces
asexually, every individual is capable of independently making offspring.
With sex, only half the individuals bear young, so the potential growth of
the population is halved. Biologists have repeatedly attempted and failed to
show what benefits of sex were sufficient to outweigh this high cost.

Researchers have agreed that the advantage of sex has to do with the
combining of genes from two mating individuals.

Those quickly created combinations, many theorized, make the organism
capable of surviving dramatic changes in the environment - a sudden
temperature rise, for example - that might drive an asexual population to
extinction.

But this process is a double-edged sword, Roughgarden noted. Sexually
produced offspring can be better suited - or less suited - to the environment
than their parents.

Roughgarden cites at least two other weaknesses in such explanations:
Biologists often have underestimated the ability of an asexual population to
generate new genetic combinations through recurring mutation, and they have
neglected the importance of ongoing, small fluctuations in the environment.

Roughgarden described his work in the October 1991 issue of The American
Naturalist. With a computer simulation, he compared two populations,
identical except for the fact that one reproduced sexually and the other
asexually. He modeled the simple case where each individual organism in
either population carried one of three possible gene combinations: aa, AA or
Aa.

He assumed that all the gene combinations needed for the populations to
survive in the fluctuating environment existed at the outset. He then
simulated environmental change and reproductive activity for 5,000
generations.

Both populations "experienced" natural selection - the process of
interaction between organisms and their environment that results in some
organisms having more offspring than others. For each generation, Roughgarden
randomly selected the strength of the environmental change and whether it was
good or bad for each gene combination or "genotype." For example, a
generation might experience an effect that allowed AA and Aa each to have two
offspring, but aa only one.

He also simulated randomly occurring mutations. In each generation, some
number of genes changed from a to A or vice versa. He randomly "mated"
individuals in the sexual population.

At the end of the 5,000 generations, he discovered that the sexual
population had maintained its numbers more successfully than had the asexual
one.

The asexual population was subject to wide swings in its numbers. An
environmental advantage to the AA genotype meant a big increase in AA
numbers, but left relatively few Aa or aa individuals to rebuild the
population when a new change wiped out most of the AAs.

"Recurring catastrophes mean the (asexual) population is always caught off
balance," Roughgarden said.

In contrast, the sexual population shuffled genes at reproduction, and
achieved a more equal distribution among genotypes for each generation. This
even distribution in turn implies a relatively consistent degree of success
from one generation to the next.

In effect, sex buffers the population from changes in the environment, he
said.

He drew an analogy to financial strategy. If a person invests $100 in two
different ways, one that earns 7 percent over two consecutive years, and one
that fluctuates from 1 percent over the first year to 13 percent over the
second, he will earn 36 cents more from the steady earner. Such small
differences, compounded over many years, can become substantial.

Long-term reproductive yield is similar: Less variance in reproduction
from generation to generation results in a higher mean performance over time,
said Roughgarden.

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